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Composite Extension
Version 0.1
2003-11-04
Keith Packard
keithp@keithp.com
1. Introduction
Many user interface operations would benefit from having pixel contents of
window hierarchies available without respect to sibling and antecedent
clipping. In addition, placing control over the composition of these pixel
contents into a final screen image in an external application will enable
a flexible system for dynamic application content presentation.
2. Acknowledgements
This small extension has been brewing for several years, contributors to
both early prototypes and the final design include:
+ Bill Haneman for motivating the ability to magnify occluded windows
with his work on accessibility
+ Carsten Haitzler for Enlightenment, the original eye-candy window
manager which demonstrated that clever hacks are an awfully
close substitute for changes in the underlying system.
+ Jim Gettys for key insights into the relationship between damage
events and per-window pixmap usage
+ Mike Harris and Owen Taylor for figuring out what to call it.
3. Architecture
The composite extension provides two related mechanisms:
1. Per-hierarchy storage. The rendering of an entire hierarchy of windows
is redirected to off-screen storage. The pixels of that hierarchy
are available whenever it is viewable. Storage is automatically
reallocated when the top level window changes size. Contents beyond
the geometry of the top window are not preserved.
2. Automatic shadow update. When a hierarchy is rendered off-screen,
the X server provides an automatic mechanism for presenting those
contents within the parent window. The implementation is free to
make this update lag behind actual rendering operations by an
unspecified amount of time. This automatic update mechanism may
be disabled so that the parent window contents can be completely
determined by an external application.
Per-hierarchy storage may be created for individual windows or for all
children of a window. Manual shadow update may be selected by only a single
application for each window; manual update may also be selected on a
per-window basis or for each child of a window. Detecting when to update
may be done with the Damage extension.
The off-screen storage includes the window contents, it's borders and the
contents of all descendants.
In automatic update mode, the X server is itself responsible for presenting
the child window contents within the parent. It seems reasonable, then, for
rendering to the parent window to be clipped so as not to interfere with any
child window content. In an environment with a mixure of manual and
automatic updating windows, rendering to the parent in the area nominally
occupied by a manual update window should be able to affect parent pixel
values in those areas, but such rendering should be clipped to automatic
update windows, and presumably to other manual update windows managed by
other applications. In any of these cases, it should be easy to ensure that
rendering has no effect on any non-redirected windows.
Instead of attempting to define new clipping modes for rendering, the
Composite extension instead defines ClipByChildren rendering to the parent
to exclude regions occupied by redirected windows (either automatic or
manual). The CreateRegionFromBorderClip request can be used along with
IncludeInferiors clipping modes to restrict manual shadow updates to the
apporpriate region of the screen. Bracketing operations with
GrabServer/UngrabServer will permit atomic sequences that can update the
screen without artifact. As all of these operations are asynchronous,
network latency should not adversely affect update latency.
4. Errors
The composite extension does not define any new errors.
5. Types
UPDATETYPE { Automatic, Manual }
6. Events
The composite extension does not define any new events.
7. Extension Initialization
The client must negotiate the version of the extension before executing
extension requests. Otherwise, the server will return BadRequest for any
operations other than QueryVersion.
QueryVersion
client-major-version: CARD32
client-minor-version: CARD32
->
major-version: CARD32
minor-version: CARD32
The client sends the highest supported version to the server and
the server sends the highest version it supports, but no higher than
the requested version. Major versions changes can introduce
incompatibilities in existing functionality, minor version
changes introduce only backward compatible changes. It is
the clients responsibility to ensure that the server supports
a version which is compatible with its expectations. Servers
are encouraged to support multiple versions of the extension.
8. Hierarchy Redirection
RedirectWindow
window: Window
update: UPDATETYPE
errors: Window, Access, Match
The hierarchy starting at 'window' is directed to off-screen
storage. 'automatic-update' specifies whether the contents
are mirrored to the parent window automatically or not. Only
one client may specify this flag, another attempt will result in an
Access error. When all clients enabling redirection terminate,
the redirection will automatically be disabled.
The root window may not be redirected. Doing so results in a Match
error.
RedirectSubwindows
window: Window
update UPDATETYPE
errors: Window, Access
Hierarchies starting at all current and future children of window
will be redirected as in RedirectWindow. If update is Manual,
then painting of the window background during window manipulation
and ClearArea requests is inhibited.
UnredirectWindow:
window: Window
errors: Window, Value
Redirection of the specified window will be terminated. If
the specified window was not selected for redirection by the
current client, a 'Value' error results.
UnredirectWindows:
window: Window
errors: Window, Value
Redirection of all children of window will be terminated. If
the specified window was not selected for sub-redirection by the
current client, a 'Value' error results.
9. Clip lists
CreateRegionFromBorderClip
region: Region
window: Window
errors: Window, IDChoice
This request creates a region containing the "usual" border clip
value; that is the area of the window clipped against siblings and
the parent. This region can be used to restrict rendering to
suitable areas while updating only a single window. The region
is copied at the moment the request is executed; future changes
to the window hierarchy will not be reflected in this region.
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